A general type of switching method using one single transformer for carrying out the switching of different power sources is illustrated in FIG. 1. In the circuit of this type, as the power from an AC power source 1 is applied on the primary coil L11 of the transformer T, power is induced in secondary coils L21-L23. The power induced in the secondary coil L21 is supplied to a regulator IC1 through a bridge type rectifier BR1 consisting of diodes D1-D4, and consequently, the regulator IC1 supplies a constant output power to the microprocessor MP regardless of change of the input load or output load. The said microprocessor has the functions of calculating the value of voltage and deciding the distribution of power to different loads 2-5, while a power source controlling signal output terminal PS supplies program signals to the base terminal of a transistor TR. Here, if a high signal from the power source controlling output terminal PS of the microprocessor MP is supplied to the base terminal of the transistor TR, then the transistor TR is turned on, and so, switching elements SD1, SD2, are also turned on. Consequently, the power induced in the secondary coils L22, L23 of the transformer T is supplied through the bridge type rectifiers BR2, BR3, the switching elements SD1, SD2, and the regulators IC2, IC3 to each of the loads 2-5. Therefore, each of the loads thus supplied with power can perform its functions.
Meanwhile, if a low signal from the power source controlling signal output terminal PS of the microprocessor MP in outputted, the transistor TR is turned off, and also the switching elements SD1, SD2 are turned off. Therefore, the power rectified in the bridge type rectifiers BR2, BR3 cannot be supplied to the regulators IC2, IC3, consequently preventing the power source from being applied to the loads 2-5.
As described above, in the conventional power switching circuit, the power source to be applied to each of loads 2 to 5 is switching over by means of the switching elements SD1, SD2, and, therefore, the volume of the switching elements, including the transistor and logic circuits, is increased. For this reason, the total circuit is accompanied by a great complication, and, furthermore, if an overvoltage or impact pulse is applied, there is the risk that the switching elements can be damaged.